This invention relates to color picture tubes of the type having a color selection electrode or shadow mask attached to a frame which is suspended in relation to a cathodoluminescent screen, and particularly to a simplified support structure for suspending the mask-frame assembly within the tube.
In these color picture tubes, the accuracy with which the electron beams strike the individual elemental cathodoluminescent screen areas depends, to a great extent, upon the accuracy with which the apertures in the shadow mask are aligned with the elemental screen areas during the operation of the tube. Thus, as the mask expands outwardly, i.e., radially, by reason of thermal effects occasioned by the impact thereon of the electron beams, the resulting misalignment of the mask apertures and elemental screen areas cause a portion of the electron beams to misregister, that is, to impinge upon elemental screen areas other than the ones upon which they are intended to impinge.
Most present day color picture tubes utilize a mask mounting assembly, such as described in U.S. Pat. No. 3,803,436, issued to Morrell on Apr. 9, 1974, to move the mask longitudinally towards the screen, as the mask is heated, to compensate for radial mask expansion. In FIGS. 1-4 of the Morrell patent, bimetallic elements are connected between studs embedded in the faceplate panel and the mask electrode. The bimetallic elements may be springs welded directly to the frame or intermediate members located between the springs and the frame. In the Morrell patent, the studs are located along the major and minor axes of the faceplate panel. Such a structure produces some instability and loss of rigidity in the mask electrode. The Morrell patent also discloses an arrangement for compensating for both radial and transverse misregister in a three-point mount rectangular tube. That structure is shown in FIGS. 14 and 15 of the Morrell patent. A more complete description of mask mounting structures and temperature compensation may be found in A. Morrell, H. Law, E. Ramberg and E. Herold, Color Television Picture Tubes, 100-102, 104-107 (Academic Press, 1974).
In a four-spring support arrangement, wherein each spring has the same orientation, e.g., all extending either clockwise or counterclockwise relative to the mask-frame assembly, thermal expansion of the springs and frame causes the mask-frame assembly to rotate about the longitudinal axis of the tube. This rotation also causes misregister of the electron beams with the elemental screen areas. The direction of rotation is determined by the location of the support structure relative to the major and minor axes of the faceplate. If the springs of the support structure are located to the right of the major and minor axes, the rotation will be oppositely directed, or clockwise. However, if the springs of the support structure are located to the left of the axes, the rotation will be counterclockwise.
A structure which corrects the aforementioned problem of rotation while providing for thermal expansion of the mask-frame assembly along the longitudinal axis is proposed in U.S. Pat. No. 4,528,475 issued on July 9, 1985, to F. R. Ragland, Jr. In the structure in that patent, an edge-to-edge bimetallic spring is angled with respect to the frame such that the angle between the spring and the frame is of an amount to align the spring-to-frame attachment point when the spring and the frame are unheated with the same attachment point when the spring and the frame are heated. In the aforementioned patent, when four studs are used to support the mask-frame assembly the studs are located adjacent to the major and minor axes of the faceplate panel. However, if the studs which support the mask-frame assembly were to be moved adjacent to the corners of the faceplate panel to increase the stability of the mask, it would be necessary to determine an angle for the springs attached along the long side of the faceplate and a different angle for the springs attached along the short side of the faceplate. The reason for the different spring angles is because of the difference in spacing from the major and minor axes to the corners of the rectangular faceplate.
Another support structure which compensates for the aforementioned problem of rotation while providing longitudinal displacement of the mask-frame assembly to correct for the thermal expansion of the mask-frame assembly is described in copending U.S. patent application Ser. No. 594,849, filed on Mar. 29, 1984, by F. R. Ragland, Jr. now U.S. Pat. No. 4,572,983. The mask-frame assembly in that application is preferably formed of cold-rolled steel which has a thermal expansion of about 33.times.10.sup.-6 centimeters per .degree.C. at 20.degree. C. The support structure disclosed therein comprises a plurality of spring assemblies, each of which includes at least two bimetal members attached between the frame of the mask-frame assembly, and a plurality of support studs affixed to the tube envelope. In the present patent application, a low expansion mask-frame assembly is used so that the support structure does not have to provide the same magnitude of thermal correction as required for mask-frame assemblies made of cold-rolled steel. Thus, a simpler support structure which provides both longitudinal and rotational compensation can be used.